Method for allocating transmission period in a wireless communication system

ABSTRACT

A method is provided for allocating a transmission period in a wireless network system. An access point (AP) transmits a PSMP frame indicating a downlink period and an uplink period allocated to each station (STA), and at least one sub PSMP frame indicating an allocated downlink period for at least one of a retransmission of downlink data and a transmission of an ACK indicating successful receipt of uplink data. After exchanging data with the AP in the downlink and uplink periods indicated by the PSMP frame, an STA receives the each sub PSMP frame, and performs at least one of a reception of the retransmitted downlink data and a reception of the ACK in the downlink period indicated by the each sub MAP frame.

PRIORITY

This application claims the benefit under 35 U.S.C. § 119(a) of anapplication entitled “Phase Resource Allocation Method” filed in theUnited States Patent and Trademark Office on Oct. 18, 2005 and assignedSer. No. 60/727,915, an application entitled “Multi-Phase ResourceAllocation Method” filed in the United States Patent and TrademarkOffice on Oct. 27, 2005 and assigned Ser. No. 60/730,924, andapplication entitled “Method and Apparatus for Allocating TransmissionPeriod in a Wireless Communication System, and System Therefor” filed inthe Korea Intellectual Property Office on Jun. 12, 2006 and assignedSer. No. 2006-52728, the entire contents of all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless communicationsystem, and in particular, to a method for efficiently allocating atransmission period in a transmission frame period of a wireless networksystem.

2. Description of the Related Art

Along with the recent development of wireless communication technologyand the resulting spread of wireless equipment, there is an increasingdemand for high-speed, highly reliable data transmission via a wirelesslink. A Wireless Local Area Network (WLAN) developed to meet the demandincludes stations (STAs), which are mobile data communication equipment,and an access point (AP) capable of exchanging data with the STAs. TheAP and the STAs, located in the same wireless service coverage area, areknown as a Base Service Set (BSS).

Particularly, an improved WLAN system achieves high throughput byemploying a Multiple Input Multiple Output (MIMO) technology that uses aplurality of transmission antennas and a plurality of receptionantennas, and an Orthogonal Frequency Division Multiplexing (OFDM)technology. In this WLAN system, STAs located in one wireless servicecoverage area transmit or receive data using wireless resourcesallocated from an AP. The AP allocates the wireless resources in theform of phase resources, and the term “phase resources” as used hereinrefers to a period in which the STAs or AP can transmit data.

FIG. 1 illustrates a structure of a transmission frame period in atypical WLAN system.

Referring to FIG. 1, in a transmission frame period 100 having a fixedlength determined in the system, an AP transmits a MAP (mapping) frame110 indicating phase resource allocation over the full transmissionframe period 100.

The MAP frame 110 is composed of a downlink MAP 120 indicating a periodin which the AP can transmit data, and an uplink MAP 122 indicating aperiod in which the STAs can transmit data. The downlink MAP 120 iscomposed of a Number-of-STAs field 130, and at least one STA Informationfield 132 determined based on the Number-of-STAs field 130. Similarly,the uplink MAP 122 is composed of a Number-of-STAs field 134, and atleast one STA Information field 136 determined based on theNumber-of-STAs field 134. The STA Information fields 132 and 136 eachinclude STA ID fields 140 and 144 that indicate STAs allocated downlinkand uplink periods, and Time Offset fields 142 and 146 that indicatetime offsets allocated to the STAs.

An STA, allocated a period in the MAP frame 110, receives data in aperiod indicated by corresponding STA information, in a downlink period112, and transmits data in a period indicated by corresponding STAinformation, in an uplink period 114. The STA maintains a sleep mode inthe other periods except for the period in which the MAP frame 110 istransmitted and the period indicated by the MAP frame 110. A contentionperiod 116 following the uplink period 114 can be accessed by an STA ona contention basis.

In this way, the AP estimates the amount of resources required for eachSTA in one transmission frame period 100, and allocates downlink anduplink periods according to the estimation. However, when the APoverestimates the amount of resources required for the STA, the wirelessresources are wasted, decreasing data throughput performance. In thiscase, the STA may not use the allocated wireless resources. Moreoverthat resource cannot be reused by other STAs since that resource isalready allocated to the particular STA. When the AP underestimates theamount of resources required for the STA, the STA cannot be allocatedits required resources until at least the next transmission frameperiod, suffering transmission delay and jitter of the uplink service.The transmission delay and jitter affects a Quality of Service (QoS)required by the STA. In addition, if the STA accesses the contentionperiod 116 due to a lack of its allocated resources, the STA cannotoperate in the sleep mode for the contention period, wasting its power.

Therefore, in the wireless communication system in which the APdetermines the downlink and uplink periods required for the STA throughscheduling, there is a need for technology to prevent the reduction indata throughput and the waste of STA power, and accurately allocate theperiods.

SUMMARY OF THE INVENTION

To substantially solve at least the above problems and/or disadvantagesand to provide at least the advantages below, the present inventionprovides a transmission period allocation method for minimizing a wasteof STA power without reducing data throughput in a wireless networksystem.

The present invention provides a method for flexibly allocating atransmission period required for an STA using more than two MAP framesin a wireless network system.

The present invention provides a method for, after allocating atransmission period to an STA using a MAP frame, allocating periodsrequired for error recovery of downlink and/or uplink data to the STAusing an additional MAP frame in a wireless network system.

According to one aspect of the present invention, there is provided amethod for receiving an allocated transmission period in a wirelessnetwork system. The method includes receiving in the transmission frameperiod a power save multi-poll (PSMP) frame providing by an access point(AP) at least one of an initial downlink period and an initial uplinkperiod; transmitting in the transmission frame period data by the AP inthe initial downlink period and receiving data by the AP in the initialuplink period; after the initial downlink period and the initial uplinkperiod, receiving in the transmission frame period a sub PSMP frameproviding an additional downlink period for retransmitting downlinkdata; and retransmitting in the transmission frame period the downlinkdata.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates a structure of a transmission frame period in aconventional WLAN system;

FIG. 2A illustrates a configuration of a WLAN system according to thepresent invention;

FIG. 2B is a block diagram illustrating an exemplary structure of an APand each STA in the WLAN system shown in FIG. 2A;

FIG. 3 illustrates a structure of a transmission frame period accordingto the present invention;

FIG. 4 is a timing diagram for a description of a power reduction effectof the phase resource allocation according to the present invention;

FIG. 5 illustrates an exemplary error recovery operation according tothe present invention;

FIG. 6 is a flowchart illustrating an operation of an AP according tothe present invention; and

FIG. 7 is a flowchart illustrating an operation of an STA according tothe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail with reference to the annexed drawings. In the followingdescription, a detailed description of known functions andconfigurations incorporated herein has been omitted for clarity andconciseness.

A main feature of the present invention, related to allocation oftransmission periods in a wireless network system, is to provide morethan one MAP (mapping) frames to indicate downlink and uplink periodsallocated by an access point (AP) to each station (STA) in atransmission frame period. The allocated period starts immediately afterthe corresponding MAP frame is transmitted, and in the transmissionframe period, the MAP frames other than the first MAP frame are called“subsequent MAPs (sub MAPs).” The sub MAP frame is used for errorrecovery of downlink and uplink data. Herein, the error recoveryincludes retransmission for the data transmitted on the downlink, andtransmission of Acknowledge (ACK) for the data received on the uplink,and can encompass other error recovery schemes.

Although a detailed description of the present invention will be madewith reference to a Wireless Local Area Network (WLAN) system based onIEEE 802.11 standards, it will be understood by those skilled in the artthat allocation of phase resources, a basic aspect of the presentinvention, can also be applied to other wireless communication systemshaving the similar technical requirements and channel formats withoutdeparting from the spirit and scope of the invention.

FIG. 2A illustrates a configuration of a WLAN system according to thepresent invention.

Referring to FIG. 2A, each of APs 202 and 210 is connected to a wirenetwork 200, and a plurality of STAs 204, 206, 208, 212 and 214 accesstheir associated APs 202 and 210 via an IEEE 802.11 Physical (PHY) layerand a wireless link based on a Media Access Control (MAC) protocol, andtransmit and receive data over a plurality of wireless channels. TheSTAs 204 to 208 and the AP 202, located in the same wireless servicecoverage 220, constitute one Base Service Set (BSS). The STAs 212 and214 and the AP 210, located in the wireless service coverage 222, formanother BSS. STAs located in each BSS can exchange data with each othervia a corresponding AP. Key functions of the APs 202 and 210 includedelivery of data traffic, access to other networks (e.g. wire network200), roaming support, synchronization in a BSS, power managementsupport, and control of media access for supporting time-bound servicein a BSS.

FIG. 2B is a block diagram illustrating an exemplary structure of an APand each STA in the WLAN system shown in FIG. 2A. Both of the AP and theSTA can include a display 232, a processor 234, a transceiver 236, aninput unit 238, a storage 240, a Random Access Memory (RAM) 242, a ReadOnly Memory (ROM) 244, and a common bus 230. The illustrated exemplarystructure is merely provided for convenience. Although specific elementsand their operations as an AP or an STA will be described herein withreference to FIG. 2B, the exemplary description should not limit thepresent invention.

Referring to FIG. 2B, the transceiver 236, connected to an antenna (notshown), receives data and converts the received signal intocorresponding digital data. The processor 234 is a controller operatingunder the control of an operating system (OS) and other programs,included in the ROM 244, and the data and information generated by theprocessor 234 is stored in the RAM 242.

Key operations of the processor 234 included in the AP includegeneration and analysis of data, allocation of periods for the STAslocated in the same wireless service coverage area and connected to theAP, generation of at least one MAP frame that indicates the allocatedperiod, and operation mode control for the transceiver 236 based on theallocated period. Specifically, the processor 234 of the AP or providesallocates initial resources R_(init) to the STAs through the MAP framelocated in the foremost part of a transmission frame. If there is a needto receive an ACK from an STA or to transmit retransmission data to theSTA, the processor 234 additionally allocates the remaining periods inthe transmission frame period to the corresponding STA, through the subMAP frame transmitted immediately after the expiration of a sequenceduration associated with the first MAP frame of the transmission frame.It would be obvious to those skilled in the art that the term“immediately after” substantially refers to a lapse of a predeterminedtime.

After expiration of the downlink/uplink period indicated by the firstMAP frame, if necessary, more than one sub MAP frames indicating thedownlink and/or uplink periods allocated in the same transmission frameperiod can be additionally transmitted. The sub MAP frame is followed bythe downlink and/or uplink periods indicated by the sub MAP frame.

Key operations of the processor 234 included in the STA includegeneration and analysis of data, generation of a transmission frame, andoperation mode control for the transceiver 236 based on a MAP framereceived from the start point of the transmission frame. The processor234 controls the transceiver 236 such that it receives the MAP framefrom the AP at the start point of every transmission frame, and analyzesthe MAP frame to determine whether its own STA ID is included therein.If its own STA ID is included in the MAP frame, the processor 234 storesin the storage 240 information on the allocated downlink and uplinkperiods indicated by the STA information associated with the STA ID, andthen wakes up the receiver and the transmitter of the transceiver 236 inthe downlink and uplink periods, respectively, so as to receive and/ortransmit data. The receiver and the transmitter of the transceiver 236enter the sleep mode in the other periods except for the allocatedperiods.

If there is an error in the data received from the AP, or if the STA isto receive an ACK for the data transmitted to the AP, the processor 234of the STA monitors whether a sub MAP frame indicating the resourcesadditionally allocated for error recovery is received from the APimmediately after expiration of the full period indicated by the MAPframe, and upon receipt of the sub MAP frame, analyzes the received subMAP frame. The full period includes all of the periods provided by theMAP frame.

According to the present invention, the data transmitted in the downlinkperiod indicated by the MAP frame is acknowledged by an ACK in thefollowing uplink period. The data transmitted in the downlink periodindicated by the sub MAP frame is acknowledged by an ACK in the uplinkperiod additionally allocated depending on the following sub MAP frame.In this case, the downlink and uplink data that requires retransmissioncan be retransmitted in the period allocated depending on the followingsub MAP frame.

The WLAN system, like the common wireless communication system, has thepossibility that errors will occur in the transmitted data due tomulti-path fading, inter-STA interference, and noise. To solve thisproblem, there have been proposed an Automatic Repeat Request (ARQ) andHybrid ARQ (H-ARQ) schemes in which a receiver sends a retransmissionrequest for failed data to a transmitter. In these schemes, the receiveruses an ACK to inform the transmitter whether there is an error in thereceived data. Upon receipt of the ACK, the transmitter determines thatthe receiver has succeeded in receiving the corresponding data (i.e.acknowledged data). However, upon failure to receive the ACK, thetransmitter determines that the receiver has failed in receiving thecorresponding data (i.e. unacknowledged data). Therefore, upon failureto receive a desired ACK, the transmitter retransmits all unacknowledgeddata.

The ARQ and H-ARQ schemes can use not only the general ACK with whichthe transmitter can be notified of the success/failure in data receptionfor individual packetized transmission data, but also a block ACK. Inthe block ACK scheme, the transmitter continuously transmits a pluralityof packets, and then, is collectively notified through a block ACKwhether the receiver has successfully received the transmitted packets.

The data transmitted/received by the STA can aggregate in one AggressiveMAC Protocol Data Unit (A-MPDU), and the A-MPDU can be transmitted forthe same downlink or uplink period, along with MAC Service Data Units(MSDUs) with different Traffic Stream Identifiers (TSIDs). The block ACKrepresents the reception result (success/failure) for a plurality ofMSDUs using a bitmap, and herein, the bitmap represents thesuccess/failure for the MSDUs succeeding the first MSDU with ‘1(success)’ or ‘0 (failure)’. The block ACK has the format and sizeoptimized according to the number of acknowledged MSDUs.

A simple block ACK can be composed of a sequence number of the firstMSDU in which a block ACK is transmitted, and the bitmap. If necessary,Multiple TID Block Acknowledge (MTBA) including a plurality of bitmapsidentified with a Traffic Identifier (TID) is used. In order to permittransmission of the block ACK having a larger size, the AP allocatesuplink or downlink periods. Herein, both the block ACK and the generalACK will be referred to as an ACK.

FIG. 3 illustrates a structure of a transmission frame period accordingto the present invention. Although it is illustrated herein that framesand uplink and downlink periods are adjacent to one another, intervalsfor transmission and reception switching and processing may exist in theactual system.

Referring to FIG. 3, in a transmission frame period 300 having a fixedlength, an AP first transmits a MAP frame 310 a with a basic rate set(including modulation scheme, coding rate, and data rate), which is lessthan a normal rate set. This is to enable all STAs in the servicecoverage area to receive the MAP frame 310 a. At the start of thecommunications, the MAP frame 310 a is periodically transmittedaccording to a fixed period (for example, about 20 ms and 100 ms forVoice over Internet Protocol (VoIP) and Moving Picture Experts Group 4(MPEG4), respectively) determined through negotiation between the AP andthe STA. The transmission frame period 300 is initialized by thetransmission of the MAP frame 310 a.

For example, the MAP frame 310 a, in order to indicate phase resourceallocation of its succeeding first downlink period 312 a and firstuplink period 314 a, is composed of a first downlink MAP 320 a thatindicates a period in which the AP can transmit data in the firstdownlink period 312 a, and a first uplink MAP 322 a that indicates aperiod in which the STA can transmit data in the first uplink period 314a. The first downlink MAP 320 a is composed of a Number-of-STAs field330 a and at least one STA Information field 332 a determined based onthe Number-of-STAs field 330 a. In the same way, the first uplink MAP322 a is composed of a Number-of-STAs field 334 a and at least one STAInformation field 336 a determined based on the Number-of-STAs field 334a.

The STA Information fields 332 a and 336 a each include STA ID fields340 a and 344 a that indicate STAs allocated the periods in the firstdownlink period 312 a and the first uplink period 314 a, Time Offsetfields 342 a and 346 a that indicate starts of the periods allocated tothe STAs, and Duration fields 343 a and 348 a that indicate lengths ofthe allocated periods. The STA ID fields 340 a and 344 a each include atleast a part of an Association Identity (AID) of each STA, or at least apart of a hardware address, i.e. MAC address, given to each STA. An STAID indicating a period for broadcast/multicast data is set to aparticular value, for example, ‘0’. The Time Offset fields 342 a and 346a each indicate an interval from the MAP frame 310 a until a start of acorresponding period in a multiple of a predetermined unit of time (forexample, 4 μs). The Duration fields 343 a and 348 a each indicate aninterval from a start to an end of the period, i.e. indicate a length ofthe period in a multiple of a predetermined unit of time (for example,16 μs).

The first downlink MAP 320 a and the first uplink MAP 322 a of the MAPframe 310 a represent downlink and uplink periods first allocated toeach STA, and can be determined according to the amount of data that theAP desires to transmit and the expected amount of data that the AP willtransmit. In this case, the AP determines the first uplink MAP 322 asuch that it includes the resources needed to send a notification of thesuccess and failure in reception of the data transmitted in the firstdownlink period 314 a. The STA, allocated a phase resource in the MAPframe 310 a, receives data in the period indicated by the correspondingSTA information 332 a in the first downlink period 312 a, and transmitsdata in the period indicated by the corresponding STA information 336 ain the first uplink period 314 a. The downlink and uplink data caninclude one or more A-MPDUs and/or MPDUs including a payload and an ACK.The STA maintains the sleep mode in the period where the MAP frame 310 ais transmitted, and the other periods except for the periods indicatedby the MAP frame 310 a.

Each STA receives data in the allocated period of the first downlinkperiod 312 a, and determines whether there is an error in the receiveddata, using an error correction code included in the received data. Forexample, each STA determines whether there is an error in each MSDUincluded in the A-MPDU. The determination result is transmitted in theallocated period of the first uplink period 314 a in the form of an ACK,along with the uplink data. In order to re-receive the failed datareceived in the first downlink period 312 a and to receive an ACK forthe data transmitted in the first uplink period 314 a, the STA monitorsthe receipt of a sub MAP frame 310 b immediately after the expiration ofall of the periods indicated by the MAP frame 310 a, i.e. downlinkperiod #1 312 a and uplink period #1 314 a. The STA transitions to anactive mode in the allocated periods of a second downlink period 312 band a second uplink period 314 b indicated by the sub MAP frame 310 b.

In order to retransmit data that was transmitted with errors in thefirst uplink period 314 a and transmit an ACK indicating the success inreceipt of data received in the first uplink period 314 a, the APallocates additional resources for error recovery, i.e. the seconddownlink period 312 b, to the STA by transmitting the sub MAP frame 310b. In this same way, the AP determines whether there is an error in thereceived data, using an error correction code included in the receiveddata. If there is an error in the data received in the first downlinkperiod 314 a, the AP can allocate the second uplink period 314 b to theSTA through the sub MAP frame 310 b in order to permit a retransmissionof the data.

That is, the sub MAP frame 310 b includes a second downlink MAP 320 band a second uplink MAP 322 b indicating phase resource allocation forthe succeeding second downlink period 312 b and second uplink period 314b. The sub MAP frame could have only one of a second downlink period anda second uplink period. However, both downlink and uplink periods foreach STA may or may not exist together. For example, if there is adownlink period, an uplink period might be needed for acknowledgementand visa versa. Similarly, the sub MAP frame 310 b can be transmittedwith the basic rate set. The second downlink MAP 320 b is composed of aNumber-of-STAs field 330 b and at least one STA Information field 332 bdetermined based on the Number-of-STAs field 330 b. The STA Informationfield 332 b includes an STA ID field 340 b that indicates an STAallocated a period in the second downlink period 312 b, a Time Offsetfield 342 b that indicates a start of the period allocated to the STA,and a Duration field 343 b that indicates a length of the allocatedperiod. The elements of the STA Information field 332 b have beendescribed above, and the second uplink MAP 322 b is equal in structureto the second downlink MAP 320 b, so a detailed description thereof willbe omitted. The STA, allocated phase resources by the sub MAP frame 310b, exchanges data and ACK with the AP in the periods indicated by thecorresponding STA information 332 b, of the second downlink period 312 band the second uplink period 314 b.

Although not illustrated, if there is still an additional need foradditional resource allocation, more than one sub MAP frames and anuplink period or a downlink period based on the sub MAP frames can beadditionally included in the transmission frame period 300. Atransmission scheme using a MAP frame capable of accompanying one ormore sub frames is referred to as a Power Save Multi-Poll (PSMP) scheme,and the MAP frame 310 a and the sub MAP frame 310 b are referred to as aPSMP frame and a sub PSMP frame, respectively. Each of the PSMP frameand the sub PSMP frame is followed by at least one downlink or uplinkperiod indicated by the corresponding (sub) PSMP frame, and one (sub)PSMP frame and a corresponding indicated period are referred to as a(sub) PSMP sequence. In other words, one PSMP sequence is initialized bytransmitting the PSMP frame, and the STA wakes up only in the periodindicated by the PSMP frame in the PSMP sequence starting with the PSMPframe, thereby minimizing power consumption.

A contention period 316 following the last sequence duration can beaccessed by a STA on a contention basis.

It should be noted that elements of the MAP frame 310 a and the sub MAPframe 310 b shown in FIG. 3 and their arrangements are subject to changewithout departing from the spirit and scope of the invention. Forexample, the MAP frame 310 a and the sub MAP frame 310 b can include oneSTA ID field, Time Offset and Duration fields for downlink transmission,and Time Offset and Duration fields for uplink transmission. In thiscase, if no period is allocated to the uplink or downlink, acorresponding Duration field is set to Null (0). In addition, the MAPframe 310 a and 310 b the sub MAP frame can include a field forindicating whether it will be followed by a sub MAP frame or another subMAP frame. The STA analyzes the field in the sub MAP frame to determinewhether the sub MAP frame is the last sub MAP frame in the currenttransmission frame period.

FIG. 4 is a timing diagram for a description of a power reduction effectof the phase resource allocation according to the present invention.

Referring to diagram (a) of FIG. 4, a transmission frame period 400includes a MAP frame 410 a, a first downlink period 412 a, a firstuplink period 414 a, a sub MAP frame 410 b, and a second downlink period412 b. The MAP frame 410 a provides phase resource allocation of thefirst downlink period 412 a and the first uplink period 414 a.

Referring to diagram (b) of FIG. 4, after switching to the sleep modeafter receipt of the MAP frame 410 a in a period 420, the STA wakes upin an allocated period 422 in the first downlink period 412 a indicatedby the MAP frame 410 a and receives downlink data from the AP. Afterswitching back to the sleep mode upon expiration of the period 422, theSTA maintains the sleep mode until an allocated period 424 in the firstuplink period 414a indicated by the MAP frame 410a. In the period 424,the STA transmits a positive ACK for the downlink data to the AP. Thepositive ACK indicates that the STA has normally received all thedownlink data. Here, the STA has no uplink data to transmit, so it hasno need to monitor the sub MAP frame 410 b, for error recovery.

Referring to diagram (c) of FIG. 4, after switching to the sleep modeafter receipt of the MAP frame 410 a in a period 430, the STA wakes upin an allocated period 432 in the first downlink period 412 a indicatedby the MAP frame 410 a, and receives downlink data from the AP. Afterswitching back to the sleep mode upon expiration of the period 432, theSTA maintains the sleep mode until an allocated period 434 in the firstuplink period 414 a indicated by the MAP frame 410 a. In the period 434,the STA transmits a negative ACK (NACK) for the downlink data to the AP,along with the uplink data. The NACK is an ACK indicating that thedownlink data is damaged. For example, the NACK is a bitmap in whichonly the bits mapped to the damaged MSDUs are set to ‘0 (failure)’.Therefore, the STA monitors receipt of the sub MAP frame 410 b in orderto be allocated the resources for retransmission of the unacknowledgeddownlink data and for reception of an ACK for the uplink data.

That is, the STA wakes up in a period 436, and monitors receipt of thesub MAP frame 410 b. Upon receipt of the sub MAP frame 410 b, the STAtransitions back to the sleep mode and waits until period 438 in asecond downlink period 414 b indicated by the sub MAP frame 410 b. Inthe period 438, the STA receives the retransmitted data and ACK from theAP. Although not illustrated, it is possible to allocate periods mappedto one or more additional MAP frames before the expiration of thetransmission frame period 400.

FIG. 5 illustrates an exemplary error recovery operation according tothe present invention. Herein, two STAs (STA 1) 504 and (STA 2) 506 arecommunicating with each other in service coverage area of an AP 502.

Referring to FIG. 5, the AP 502 transmits a MAP frame 510 a at the startof a transmission frame period. The MAP frame 510 a provides periods ina first downlink period 512 allocated to the STA1 504 and the STA2 506,and periods in a first uplink period 514 allocated to the STA1 504 andthe STA2. 506. After transmitting the MAP frame 510 a, the AP 502 firsttransmits downlink data 512 a to the STA1 504, and thereafter, transmitsdownlink data 512 b to the STA2 506. Herein, the downlink data 512 btransmitted to the STA2 506 is partially damaged during transmission.

Upon receipt of the downlink data 512 a from the AP 502, the STA1 504transmits uplink data 522 a and a positive ACK 522 b for the downlinkdata 512 a to the AP 502 in a period 522 of the first uplink period 514indicated by the MAP frame 510 a. Here, the STA1 504, as it hasremaining queued data 524 left therein, transmits data in the period522, together with a Resource Request (RR) message for requesting theadditional resources required for transmitting the queued data 524.

After receiving the failed downlink data 512 b from the AP 502, the STA2506 transmits to the AP 502 uplink data 532 a and a negative ACK 532 bfor the downlink data 512 b in a period 532 of the first uplink period514 indicated by the MAP frame 510. No RR message is sent by the STA2506 in the period 532 since it did not have any remaining queued data.The uplink data 532 a originating from the STA2 506 is partially damagedduring transmission.

In response to the uplink data 522 a including the RR message from theSTA1 504 and the damaged uplink data 532 a and the NACK 532 b from theSTA2 506, a sub MAP frame 510 b indicates an downlink period for an ACK516a to the STA1 504 and retransmission data 516 b and a NACK 516 c tothe STA2 506 in a second downlink period 516, a period 526 for uplinkdata 526 a from the STA1 504, and a period 534 for uplink data 534 a andan ACK 534 b from the STA2 506. Therefore, in the periods indicated bythe sub MAP frame 510 b, the AP 502 exchanges data and control message(ACK) with the STAs 504 and 506. That is, if the AP 502 receives a NACKfor the downlink data 512 b transmitted to the STA2 506, or at leastfails to receive an ACK, the AP 502 allocates resources forretransmission of the downlink data 512 b through the sub MAP frame 510b.

The AP 502, as it received the uplink data 526 a and 534 a from the STAs504 and 506, allocates a period 518 for transmission of ACKs 518 a and518 b for the uplink data 526 a and 534 a through a second sub MAP frame510 c. The STAs 504 and 506 receive ACKs 518 a and 518 b for the uplinkdata 526 a and 534 a in the period 518 indicated by the second sub MAPframe 510 c. The MAP frame 310 a, the first sub MAP frame 510 b, thesecond sub MAP frame 510 c, and all of the downlink and uplink periodsassociated therewith all occur within one transmission frame period 500.

FIG. 6 is a flowchart illustrating an operation of an AP according tothe present invention. Herein, for simplicity purposes the APcommunicates with only one STA.

Referring to FIG. 6, in step 602, the AP initializes the currenttransmission frame period by transmitting a MAP frame indicating aperiod allocated to the current transmission frame period. In step 604,the AP transmits downlink data to an STA for a period indicated by theMAP frame. If there is no downlink data to transmit to the STA, the MAPframe includes no downlink period to be allocated to the STA. In thiscase, step 604 is omitted.

In step 606, the AP determines whether uplink data is received from theSTA for the period indicated by the MAP frame. If there is no periodallocated to the STA by the MAP frame, the AP proceeds to step 610.However, if uplink data is received from the STA, the AP proceeds tostep 608 where it allocates a downlink period for transmission of an ACKfor the uplink data, then proceeds to step 610.

In step 610, the AP determines whether an ACK for the downlink datatransmitted in step 604 was received in step 606. If the received ACK isa positive ACK, the AP proceeds to step 614. However, if the receivedACK is not the positive ACK but a negative NACK, the AP allocates adownlink period for retransmission of the downlink data in step 612, andthen proceeds to step 614.

In step 614, the AP determines whether an RR message was received fromthe STA in step 606. If no RR message was received, the AP proceeds tostep 618, determining that there is no need for additional resourceallocation in the current transmission frame period. However, if the RRmessage was received in step 614, the AP allocates a resource, i.e. anuplink period, requested by the RR message in step 616, and thenproceeds to step 618.

In step 618, the AP determines whether there is any resource to beadditionally allocated in the current transmission frame period. Ifresource was allocated in any one of steps 608, 612 and 616, the APproceeds to step 620 where it generates a sub MAP frame indicating theresource allocated in at least one of steps 608, 612 and 616, transmitsthe sub MAP frame immediately after expiration of the periods indicatedby the MAP frame, and then returns to step 604. However, if there is noresource to be additionally allocated in the current transmission frameperiod, the AP proceeds to step 622 where it waits until a start pointof the next transmission frame period, i.e. the next transmission periodof the MAP frame, and then returns to step 602.

FIG. 7 is a flowchart illustrating an operation of an STA according tothe present invention.

Referring to FIG. 7, in step 702, the STA, upon receipt of a MAP framefrom an AP, recognizes a start of the current transmission frame periodand detects a downlink period and/or an uplink period allocated thereto.If the downlink period is indicated by the MAP frame, the STA determinesin step 704 whether downlink data was successfully received in thedownlink period. If no downlink period is indicated, the STA proceeds tostep 706. If the downlink data was not successfully received, the STAproceeds to step 714 where it prepares to receive a sub MAP frame so asto be allocated a downlink period for re-reception of the downlink data,and then proceeds to step 706. Otherwise, the STA directly proceeds tostep 706. At this moment, an ACK for the downlink data is transmitted inthe succeeding uplink period.

In step 706, the STA determines whether there is a need to receive anACK for the uplink data transmitted in the uplink period allocated bythe MAP frame. If the STA transmitted the uplink data, the STA proceedsto step 716 where prepares to receive a sub MAP frame so as to beallocated a downlink period for reception of an ACK for the uplink data,and then proceeds to step 708. Otherwise, the STA directly proceeds tostep 708.

In step 708, the STA determines whether an RR message was transmitted inthe uplink period allocated by the MAP frame. If the RR message wastransmitted, the STA proceeds to step 718 where it prepares to receive asub MAP frame so as to be allocated the resource requested by the RRmessage, and then proceeds to step 710. Otherwise, the STA directlyproceeds to step 710.

In step 710, the STA determines whether the reception of the sub MAPframe is set up, i.e. whether it should re-receive downlink data, itshould receive an ACK for the uplink data, or it requests additionalresources. If the reception of the sub MAP frame is not set up, the STAwaits until a start point of the next transmission frame period, i.e.the next transmission period of the MAP frame in step 712, determiningthat there is no need for additional resources in the currenttransmission frame period, and then returns to step 702. However, if thereception of the sub MAP frame is set up, the STA proceeds to step 720.

In step 720, the STA monitors reception of the sub MAP frame from the APimmediately after expiration of the full period indicated by the MAPframe. Upon receipt of the sub MAP frame, the STA returns to step 704and performs the above processes in the succeeding periods indicated bythe sub MAP frame.

As can be understood from the foregoing description, the AP allocatesonly the minimum resources through the first MAP frame to minimize anyoverestimation of resources, and if the STA has a need for additionalresources, the AP immediately allocates through the sub MAP frame theuplink resources that are requested in the same transmission frameperiod, thereby removing the QoS reduction cause such as the delay andjitter. In addition, the STA maximizes the sleep mode holding timewithout unnecessarily monitoring the long period, thereby reducing itspower consumption.

While the invention has been shown and described with reference to acertain preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method for receiving an allocated transmission period in atransmission frame period having a preset duration in a wireless networksystem in a wireless network system, the method comprising the steps of:receiving in the transmission frame period a power save multi-poll(PSMP) frame providing by an access point (AP) at least one of aninitial downlink period and an initial uplink period; transmitting inthe transmission frame period data by the AP in the initial downlinkperiod and receiving data by the AP in the initial uplink period; afterthe initial downlink period and the initial uplink period, receiving inthe transmission frame period a sub PSMP frame providing an additionalperiod for retransmitting at least one of downlink data and uplink data;and retransmitting in the transmission frame period at least one of thedownlink data and the uplink data.
 2. The method of claim 1, whereinmore than one sub PSMP frame can be transmitted within one transmissionframe period.
 3. The method of claim 1, wherein an acknowledgement (ACK)indicating successful receipt of data received in the downlink periodindicated by the MAP frame is transmitted to the AP in the initialuplink period.
 4. The method of claim 1, wherein the ACK includes abitmap indicating successful receipt of at least one data unit.
 5. Themethod of claim 1, wherein the sub PSMP frame provides an additionaluplink period allocated for a retransmission of uplink data that failedin an uplink period immediately before transmission of the correspondingsub PSMP frame.
 6. The method of claim 1, further comprising monitoringfor the sub PSMP frame, if there is an error in the receipt of downlinkdata received in a downlink period immediately before a start of acorresponding sub PSMP frame, or if there is uplink data transmitted inan uplink period immediately before the start of the corresponding subPSMP frame.
 7. The method of claim 1, wherein the sub PSMP framecomprises: an identifier (ID) of a station (STA); a start offsetindicating a start point of each period provided by the sub PSMP frame;and a duration field indicating a length of each period provided by thesub PSMP frame.
 8. The method of claim 1, wherein all of the periodsprovided by the PSMP frame and all of the periods provided by the subPSMP frame are included in one transmission frame period having a fixedlength.